1. Field
This invention relates generally to the field of optical measurement instruments, such as eye examination instruments, and more particularly an apparatus and method for measuring or computing the distance from an object under test (e.g., an eye) to an optical or mechanical reference plane of a measurement instrument.
2. Description
Optical measurement instruments, such as instruments which measure the optical properties of the human eye, rely on the correct working distance being set between a reference plane of the instrument and a reference plane of the object being measured, so that the best possible imaging conditions are met, resulting in the highest possible accuracy of the instrument. A misalignment of this distance (hereinafter referred to as the “Z-Distance”) can result in measurement inaccuracies. In the case of corneal topography, keratometry and wavefront aberration measurements the result can be a misinterpretation of the radius of curvature of either a surface or waves of light, or error in measurement of other parameters such as ocular refraction, higher order aberrations or pupil size. These inaccuracies will furthermore be of unknown magnitude, due to the unknown error in the Z-distance.
Accordingly, it would be advantageous to provide an arrangement that can be used to determine accurately the distance between a reference plane of an optical measurement instrument and a reference plane of an object under test. It would further be advantageous to provide such an arrangement that can be in a standalone configuration, or integrated into the optical measurement instrument.
It would also be advantageous to provide a method for accurately measuring or computing the distance from a surface of an eye or other object under test to an optical or mechanical reference plane of a measurement instrument. Other and further objects and advantages will appear hereinafter.
In one aspect of the invention, a method is provided for measuring the distance from a reference plane of an optical measurement instrument to a reference plane of an object under test. The method comprises: illuminating the object under test with a source; passing light from the object under test through an optical system to produce an aberrated image of the source such that an aspect of the aberrated image varies when the distance between the reference plane of the object and the reference plane the optical system changes; sensing the aberrated image with an optical sensor; and determining the distance from the reference plane of the optical measurement instrument to the reference plane of the object under test based on an aspect of an aberrated image sensed by the optical sensor (e.g., based on a shape, aspect ratio, or intensity distribution of the aberrated image, or some portion thereof). In some embodiments, the aspect of the aberrated image varies between when the distance is greater than a nominal distance DNOM, and when the distance is less than DNOM, the variation being different in a first direction than in a second direction perpendicular to the first direction. In some embodiments, the source is dimensioned to form or approximate a point source. In other embodiments, the method includes illuminating the test object with a light pattern comprising two or more sources.
In another aspect of the invention, a system is provided for measuring a distance from a reference plane of an optical measurement instrument to a reference plane of an optical device under test. The system comprises: an illumination system adapted to illuminate the object under test; an optical system adapted to receive light from the object under test and to produce an aberrated image; an optical sensor adapted to receive and sense the aberrated image; and a processor adapted to determine the distance from the reference plane of the optical measurement instrument to the reference plane of the optical device based on an aspect of the aberrated image sensed by the optical sensor (e.g., based on a shape, aspect ratio, or intensity distribution of the aberrated image, or some portion thereof). In some embodiments, the aberrated image is formed such that an aspect of the aberrated image varies between when the distance is greater than a nominal distance DNOM, and when the distance is less than DNOM, the variation being different in a first direction than in a second direction perpendicular to the first direction. In some embodiments, the illumination system comprises source that is dimensioned to form or approximate a point source. In other embodiments, the illumination system comprises a light pattern having two or more sources.
In yet another aspect of the invention, a system is provided for measuring a distance from a reference plane of an optical measurement instrument to a reference plane of an object under test. The system comprises: an illumination system for providing a pattern of light to the object under test; an optical system which forms an astigmatic image of the pattern of light; an optical sensor adapted to receive the astigmatic image; and a processor adapted to determine the distance from the reference plane of the optical measurement instrument to the reference plane of the optical device based on an aspect of the astigmatic image sensed by the optical sensor (e.g., based on a shape, aspect ratio, or intensity distribution of the aberrated image, or some portion thereof).